As we sleep, speedy brain waves boost our ability to learn

Scientists have long puzzled over the many
hours we spend in light, dreamless slumber. But a new study from the University of California, Berkeley, suggests
we're busy recharging our brain's learning capacity during this traditionally undervalued phase of sleep, which can take up
half the night.

UC Berkeley researchers have found compelling evidence that bursts of brain
waves known as "sleep spindles" may be networking between key regions of the brain to clear a path to
learning. These electrical impulses help to shift fact-based memories from the brain's hippocampus – which has
limited storage space – to the prefrontal cortex's "hard drive," thus freeing up the hippocampus to take in fresh
data. Spindles are fast pulses of electricity generated during non-REM sleep, and they can occur up to 1,000 times a
night.

"All these pieces of the puzzle tell a consistent and compelling story – that sleep
spindles predict learning refreshment," said Matthew Walker, associate professor of psychology and neuroscience
at UC Berkeley and senior author of the study to be published March 8 in the journal Current
Biology.

The study found that this spindle-driven networking was most likely to happen during
Stage 2 of non-Rapid Eye Movement (NREM) sleep, which occurs before we reach the deepest NREM sleep and the dream
state known as Rapid Eye Movement (REM) sleep. This shallow stage of dreamless slumber can account for half our sleeping
hours, and happens most frequently during the second half of the night, or in the latter part of a period in which we
sleep.

"A lot of that spindle-rich sleep is occurring the second half of the night, so if you
sleep six hours or less, you are shortchanging yourself. You will have fewer spindles, and you might not be able to
learn as much," said Bryce Mander, a post-doctoral fellow in psychology at UC Berkeley and lead author of the
study.

As for broader societal ramifications, researchers said evidence that brain waves during the
latter part of the sleep period promote our capacity to store fact-based memories raises the question of whether the
early school day is optimal for learning.

"These findings further highlight the importance of sleep in our
educational populations, where the need for learning is great, yet late bedtimes and early school start times prevent
adequate sleep amounts," Mander said.

On average, adults spend one-third of their lives sleeping. Yet,
no scientific consensus has been reached on why humans need sleep. Walker said. Previous research led
by Walker has shown that a good night's rest helps us regulate our moods and cope with emotional challenges, while sleep
deprivation can make otherwise reasonable people emotionally shaky, indicating a strong correlation between sleep loss and
psychiatric disorders.

For this latest study, Walker and his team took 44 healthy young adults and subjected
them to a rigorous memorizing task intended to tax the hippocampus. All participants performed at similar levels. The group
was then divided, with one half taking a 90-minute nap while the other half stayed awake.

That evening, the
entire group was subjected to another round of learning. The ability to memorize new information deteriorated for those who
had remained awake throughout the day. In contrast, those who had napped not only performed better than the waking group, but
actually improved their capability for learning, as if sleep had refreshed their memory capacity, the study
found.

Electroencephalogram tests, which measured electrical activity in the brains of the nappers, showed that
the more sleep spindles the nappers produced, the more refreshed they were for learning. Furthermore,
researchers were able to link sleep spindles to brain activity looping between the lobes of the brain that house the
hippocampus and prefrontal cortex – two critical areas for memory.

"Our findings demonstrate that sleep
may selectively seek out and operate on our memory systems to restore their critical functions," Walker
said.

"This discovery indicates that we not only need sleep after learning to consolidate what
we've memorized, but that we also need it before learning, so that we can recharge and soak up new information the next
day."

UC Berkeley psychology students Sangeetha Santhanam and Jared M. Saletin
are co-investigators in the study.